Protein kinases are a group of enzymes that regulate diverse, important biological processes including cell growth, proliferation, survival, invasion and differentiation, organ formation, tissue repair and regeneration, etc. Protein kinases exert their physiological functions through catalyzing the phosphorylation of protein and thereby modulating the cellular activities. Because protein kinases have profound effects on cells, their activities are highly regulated. Kinases are turned on or off by phosphorylation (sometimes by autophosphorylation), by binding of activator proteins or inhibitor proteins, or small molecules, or by controlling their location in the cell relative to their substrates. Dysfunctions in the activities of kinases, arising from genetic abnormalities or environmental factors, are known to be associated with many diseases. Several severe pathological states, including cancer and chronic inflammation, are associated with stimulation of intra-cellular signaling, and since kinases positively relay signaling events, their inhibition offers a powerful way to inhibit or control signal transduction cascades.
The epidermal growth factor receptor (EGFR; ErbB-1; HER1 in humans) is a member of the ErbB family of receptors, a subfamily of four closely related receptor tyrosine kinases: EGFR (ErbB-1), HER2/c-neu (ErbB-2), Her 3 (ErbB-3) and Her 4 (ErbB-4). EGFR is the cell-surface receptor for members of the epidermal growth factor family (EGF-family) of extracellular protein ligands. Mutations affecting EGFR expression or activity could result in cancer. EGFR is reported deregulated in most solid tumor types i.e. lung cancer, breast cancer and brain tumor. It is estimated that mutations, amplifications or misregulations of EGFR or family members are implicated in about 30% of all epithelial cancers. Therapeutic approaches have been developed based on the inhibition of EGFR by either antibody drug or small molecular inhibitor drug, such as gefitinib and erlotinib. In the case of non-small cell lung cancer, gefitinib and erlotinib have shown benefit for about 10-40% of the patients. However, acquired resistant to gefitinib or erlotinib after a period of treatment become a major clinical problem. Research has confirmed that one main reason resistance developed is due to the presence of a new mutation of T790M, which is the gatekeeper of EGFR. Subsequently, inhibitors can overcome this T790M have been developed and showed advantage in the clinical trial, such as BIBW2992. However, these T790M targeted EGFR inhibitor still has relative inhibitory activity towards wild type EGFR which limit the clinical application. It is needed to further develop more efficient type of EGFR inhibitor which will target substantially the mutation and not substantially the wild type protein.
Other protein kinases that are useful targets for small molecule pharmaceuticals include B lymphoid tyrosine kinase (BLK), janus kinase 1 (JAK1), bone marrow kinase on the X chromosome (BMX/ETK), Bruton's tyrosine kinase (BTK), janus kinase 2 (JAK2), janus kinase 3 (JAK3), tyrosine kinase expressed in hepatocellular carcinoma (TEC), resting lymphocyte kinase (TXK, also known as RLK), FMS-like tyrosine kinase 3 (FLT3), and FLT3 (D835Y).